In recent years, a magnetic bearing has been developed as a substitute for a contact type bearing. Since this magnetic bearing supports a rotor in a non-contact state, a friction coefficient becomes substantially zero, thereby enabling high-speed rotation. Further, since this magnetic bearing does not require a lubricating oil, it can be used in a special environment such as a high-temperature atmosphere, a low-temperature atmosphere or a vacuum, and it does not need maintenance. Thus, using this magnetic bearing to support a rotor of a motor has been considered.
For example, as shown in FIG. 10, there has been developed a magnetic levitation motor 107 comprising: a rotor 101 being a magnetic substance surrounding a stator core 102; motor magnets 103 arranged and fixed to inner peripheral surfaces of both end portions of the rotor 101 so as to be opposed to an outer peripheral surface of the stator core 102; direct-current magnetic field generating means 105 each of which is provided on a stator 109 side and is a permanent magnet which generates a direct-current magnetic field (bias magnetic field) 104 which spreads in a radial pattern; a first stator winding 106 which generates a levitation control magnetic flux (not shown) which controls levitation of the rotor 101 in a radial direction; and a second stator winding 111 which generates a rotating magnetic field to rotate the rotor 101 (see Japanese patent application laid-open No. 2000-184655). It is to be noted that the first stator winding 106 and the second stator winding 111 are illustrated as one coil in FIG. 10 in order to simplify the drawing, but they are actually different coils. The first stator winding 106 is wound as shown in FIG. 11. It is to be noted that FIG. 11 shows only the stator winding in a direction y. Actually, besides the illustrated stator winding, a stator winding in a direction x and respective motor windings having phases U, V and Z are provided.
In this magnetic levitation motor 107, a regular brushless motor and a magnetic bearing are simultaneously constituted between the rotor 101 and the stator 109. Furthermore, the rotor 101 is levitated in the radial direction by a bias magnetic flux generated by the direct-current magnetic field generating means 105 and a levitation control magnetic flux (not shown) generated by the first stator winding 106. Moreover, the rotor 101 is levitated in an axial direction by a thrust bearing winding 108 provided to the stator 109. Here, the stator core 102 and the motor magnet 103 opposed thereto have the same thickness in the axial direction.
In the above-described magnetic levitation motor 107, however, since the motor magnet 103 and the stator core 102 have the same thickness in the axial direction, the bias magnetic flux 104 and the levitation control magnetic flux (not shown) necessarily go through the motor magnet 103, and a magnetic resistance becomes large when forming a magnetic circuit of the bias magnetic flux 104 and the levitation control magnetic flux. That is because an air gap G between the rotor 101 and the stator core 102 cannot be sufficiently reduced since the motor magnets 103 are interposed between the rotor 101 and the stator core 102.
Therefore, the sufficient bias magnetic flux 104 and levitation control magnetic flux cannot be generated, and it is difficult to increase a levitation force of the rotor 101 in the radial direction. In particular, when the magnetic levitation motor 107 is reduced in size, these problems prominently appear.
It is, therefore, an object of the present invention to provide a magnetic levitation motor which can increase a levitation force of a rotor in a radial direction.